System and process for aluminization of metal-containing substrates

ABSTRACT

A system and method are detailed for aluminizing surfaces of metallic substrates, parts, and components with a protective alumina layer in-situ. Aluminum (Al) foil sandwiched between the metallic components and a refractory material when heated in an oxidizing gas under a compression load at a selected temperature forms the protective alumina coating on the surface of the metallic components. The alumina coating minimizes evaporation of volatile metals from the metallic substrates, parts, and components in assembled devices during operation at high temperature that can degrade performance.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a non-provisional application that claims priority from U.S.Provisional Patent Application No. 61/683,489 filed 15 Aug. 2012entitled “Process for Metalizing Surfaces In-Situ”, which reference isincorporated herein in its entirety.

STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under ContractDE-ACO5-76RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to systems and methods forcoating metals. More particularly, the invention relates to a system andprocesses for aluminizing metallic substrates and components.

BACKGROUND OF THE INVENTION

Oxidation resistant ferritic stainless steels are considered to bepromising candidate materials for interconnects and cell frames in hightemperature electrochemical devices including solid oxide fuel cells(SOFC) and solid oxide electrolysis cells (SOEC) interconnectapplications in SOFC stacks operating in the intermediate temperaturerange of from about 650° C. to about 850° C. due to their thermalexpansion match with other stack materials (e.g., anode-supported cellsand seals), their ability to form a conductive oxide scale, and theirrelatively low cost. However, these metals require a protective coatingto block evaporation of chromium (Cr), an important constituent of themetals. Without the protective coating, volatile chromium (Cr) speciescan evaporate and poison the electrochemical cell thereby degrading theelectrochemical performance over time. Aluminization is considered aviable solution to address the evaporation problem in ferritic steelcomponents. Aluminization is conventionally performed with suchhigh-temperature processes as vapor phase deposition and packcementation. However, these conventional approaches must be performed onmetallic components before being inserted in the stack during the stackassembly, and often involve expensive aluminum precursor materials. Insome cases, an additional high-temperature heat treatment may be neededto re-flatten individually aluminized components to eliminate anywarping that occurred during the aluminization process. As will beappreciated by those of ordinary skill in the art, extra processingsteps can increase manufacturing costs. Accordingly new processes areneeded that can provide aluminization of metallic parts economically,efficiently, and without the need for these heat treatments and/orexpensive raw material. The present invention addresses these needs.

SUMMARY OF THE INVENTION

The present invention includes a method for aluminizing metal-containingsubstrates. The method may include compressing an aluminum foil of aselected thickness between one or more metal-containing substrates and arefractory material of a selected thickness with at least onecompression component under a selected compression load in an oxidizinggas at a selected temperature for a time sufficient to form an aluminumoxide coating layer on the surface of the metal-containing substrates.The aluminum oxide coating layer on the surface of the metal-containingsubstrates prevents, reduces, and/or minimizes release of volatilemetals including chromium (Cr) from the metal-containing substrates thatcan reduce performance of the device during operation at an operationtemperature, as compared to metal-containing substrates that do notinclude the aluminum oxide coating layer. The aluminum oxide coatinglayer does not spall from the surface of the metal-containing substratesin a device during operation at the operating temperature of the device.The present invention eliminates need for separate heat treatments orpost-firing heat and cleaning treatments. As such, the process is moreeconomical and more efficient than conventional coating technologies.

The present invention also includes a system for aluminizing a surfaceof metal-containing substrates. The system may include: a compressionassembly that includes at least one compression component configured tocompress an aluminum foil of a selected thickness between one or moremetal-containing substrates and a refractory material of a selectedthickness under a selected compression load in an oxidizing gas at aselected temperature for a time sufficient to form an aluminum oxidecoating on the surface of the metal-containing substrates.Metal-containing substrates may include a metal alloy such as ferriticstainless steel, but metal-containing substrates are not limited. Insome applications, metal-containing substrates may be components of hightemperature electrochemical devices such as solid oxide fuel cells.

In some applications, the surface of the metal-containing substrates maybe a flat surface. In some applications, the surface of the substrate ormetal component may be other than a flat surface.

The aluminum (Al) foil may be of various selected thicknesses. Invarious applications, thickness of the aluminum foil may be betweenabout 0.001 mm and about 0.5 mm. In some applications, the aluminum foilmay include a thickness of less than about 25 μm (microns).

Aluminization may be performed absent annealing at a selectedtemperature.

The refractory material of the present invention may be a sheet of mica.Thickness of the mica sheet is not limited. In some applications, thesheet of mica may include a thickness of, e.g., 0.3 mm (0.012 inches).

Aluminization of the metal-containing substrates may be performed withinan assembled device in-situ. Devices are not limited. Exemplary devicesare high temperature electrochemical devices including, e.g., solidoxide fuel cells (SOFCs), solid oxide electrolyzer cells (SOECs), oxygenmembranes, and other devices. In some applications, the assembled devicemay be a stack assembly of an electrochemical device and themetal-containing substrates may be components of the stack assemblyincluding, e.g., interconnects, frames of ceramic cells, or combinationsof these components. In some applications, aluminization ofmetal-containing components of an electrochemical device includinginterconnects and cell frames may be performed in-situ, e.g., duringstack fabrication heat treatment after assembly of the stack components.Here, the aluminizing heat treatment may be identical to the heattreatment used to bond individual stack components together after astandard stack assembly. The instant approach eliminates need for apreliminary aluminizing heat treatment of individual metallic componentsprior to stack assembly.

In some applications, aluminization of the metal-containing componentsof the electrochemical device may be performed prior to assembly in theelectrochemical device. For example, aluminization may be performed onindividual interconnects and cell frames of an electrochemical deviceprior to assembling the individual components in a stack assembly.

Compression components of the compression assembly may includecompression plates constructed of a high-temperature refractory materialincluding ceramic blocks or metals that deliver the compression loaduniformly through the aluminum foil to the metal-containing substrates.

In some applications, compression loads may be provided byhigh-temperature compression discs, or external high-load compressionsprings. Compression loads may be delivered to the assembled device withsuch devices as high-temperature compression discs, or externalhigh-load compression springs. In various applications, the compressionload may be greater than or equal to about 6800 Newtons per square meter(N/m²). The compression load may be applied for a time greater than orequal to about 5 minutes at the selected temperature.

Heating of the metal-containing substrates may be performed, e.g., in afurnace or a heater. Heating may include diffusing aluminum from thealuminum foil into the surface of the metal-containing substrate,resulting in the formation of the aluminum oxide coating layer on thesurface of the metal-containing substrates. Heating temperature may beselected between about 660° C. and about 1200° C. In some applications,reaching the selected heating temperature may include a heating ratebetween about 1° C. per minute and about 10° C. per minute. Oxidizinggases may include air, or gas mixtures that include air.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especially thescientists, engineers, and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system for aluminizing metal-containing parts andcomponents, according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view of a system for aluminizingmetal-containing substrates of an assembled device in-situ, according toan embodiment of the present invention.

FIG. 3 is an electromicrograph showing an alumina coating layer formedon a surface of a metal-containing substrate in accordance with thepresent invention.

FIG. 4 is an electromicrograph showing an alumina coating layer formedon a surface of another metal-containing substrate in accordance withthe present invention.

DETAILED DESCRIPTION

A system and method are detailed for coating metallic parts andmetal-containing substrates with a protective alumina layer thateliminates need for post-coating treatments including, e.g.,post-coating firing or other post treatment steps. The process issimple, efficient, and economical compared with conventionalaluminization processes. The following description includes a preferredbest mode of one embodiment of the present invention. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims. Therefore the presentdescription should be seen as illustrative and not limiting.

FIG. 1 shows a compression assembly 100 for aluminizing metal-containingparts and components. “Aluminization” as the term is used herein meansto coat metallic surfaces of metal-containing substrates with analuminum oxide (alumina) coating. Compression assembly 100 includesaluminum foil 4 of a selected thickness that is sandwiched between ametal-containing substrate 2 composed of a selected metal or metalalloy, and a refractory 6 composed, e.g., of a sheet of mica. Thicknessof mica sheet 6 and aluminum foil 4 are not limited. In someembodiments, substrate 2 may include, or be composed of, e.g., ferriticstainless steel (e.g., grade AISI441 ferritic stainless steel) oranother suitable metal. Compression assembly 100 may include acompression plate 8 that maintains contact between aluminum foil 4 andthe surface of substrate 2 and further delivers a compression load 10directly and uniformly through aluminum foil 4 that facilitatesaluminization of the surface of substrate 2. The aluminum (Al) foil mayinclude selected thicknesses. In various embodiments, the aluminum foilmay include a thickness of between about 0.001 mm and about 0.5 mm. Insome embodiments, the aluminum foil may include a thickness of less thanabout 25 μm (microns).

Compression assembly 100 can deliver a compression load required foraluminization of the components in electrochemical devices (e.g., SOFCdevices) and simultaneously serve as a hybrid compression seal foroperation of the electrochemical devices. In some embodiments, thecompression load may be greater than or equal to about 6800 Newtons/m².Compression loads may be provided by components in the electrochemicaldevice assembly such as high-temperature compression discs, or via suchdevices as external high-load compression springs. However, allcompression devices as will be implemented by those of ordinary skill inthe art in view of the disclosure are within the scope of the invention.No limitations are intended. In some embodiments, the compression loadmay be applied for a time greater than or equal to about 5 minutes atthe selected temperature.

In an exemplary embodiment, the mica sheet 6 may include a thickness ofabout 0.012 inches (0.3 mm). But, thickness dimensions are not intendedto be limited. When assembled into the electrochemical device assembly,mica sheet 6 may be heated at a temperature of, e.g., 550° C. for a timesufficient to burn off organic binders in the mica sheet that may bedetrimental to operation of the electrochemical device in which it isintroduced. Times for removing binders are not limited. Typical timesfor removing organic binders may be up to about 2 hours, but times andtemperatures are not intended to be limited. Alternatively, mica sheetsfree of organic binders may be employed.

Compression assembly 100 may be heated, e.g., in a furnace or heater 12in an oxidative gas such as air at the selected aluminizationtemperature for a time sufficient to aluminize the surface of thesubstrate 2. Temperatures for aluminization are not limited. In someembodiments, aluminization temperature may be, e.g., about. 900° C. Invarious embodiments, aluminization temperature may be between about 660°C. and about 1000° C. In some embodiments, heating the substrate(including parts or components) to the aluminization temperature mayinclude a heating rate of between about 1° C. per minute and about 10°C. per minute.

Times to effect aluminization are also not limited. In some embodiments,aluminization can be completed by heating substrate for a time greaterthan or equal to about 5 minutes at the selected aluminizationtemperature. In some embodiments, aluminization can be completed byheating the substrate at the aluminization temperature for about 2 hourson average.

Aluminum oxide coatings on aluminized metallic substrates obtained inconcert with the present invention minimize release of volatile metalspecies including chromium (Cr) from metallic substrates in assembledelectrochemical devices during operation at elevated temperatures thatcan poison and degrade performance in these electrochemical devices andcells.

In-Situ Aluminization in Assembled Devices

The present invention can be used to aluminize metal-containingsubstrates in assembled devices in-situ including metallic parts thatcontain stainless steel or other metal alloys comprising chromium (Cr)in high temperature electrochemical devices. Devices are not limited.Exemplary devices include high temperature electrochemical devices. Hightemperature electrochemical devices include, but are not limited to,e.g., solid oxide fuel cells (SOFCs), solid oxide electrolyzer cells(SOECs), oxygen membranes, and other devices. While aluminization offlat substrates, parts, and components is described hereafter, theinvention is not limited to flat substrates, parts, and components only.FIG. 2 shows a cross-sectional view of a compression assembly 100configured for aluminizing metallic components of an exemplary planarsolid oxide fuel cell (SOFC) 50 in-situ, in accordance with the presentinvention. The instant configuration also thermalizes device componentsin preparation for thermal cycling during operation of SOFC 50. As shownin the figure, aluminum (Al) foil 4 may be applied to surfaces ofcathode interconnect 22, anode interconnect 24, window (stack) frame 26,and/or other metallic components of SOFC 50. In the instant embodiment,mica paper 6 may be positioned between respective sheets or layers ofaluminum foil 4 positioned on the cathode side of ceramic(electrochemical) cell 28 to maintain good contact between the layers ofaluminum foil 4 on an inner side of interconnect 22 and window frame 26to maintain good contact between the layers of aluminum foil 4, and on aside of interconnect 24 and window frame 26 on the anode side of ceramic(electrochemical) cell 28, respectively. Compression (load carrying)plates 8 may be positioned on an exterior side of cathode interconnect22 and an exterior side of anode interconnect 24 that deliverscompression load 10 to respective sides of electrochemical device 50mounted in compression assembly 100. Metallic components ofelectrochemical device 50 may be aluminized by introducing an oxidizinggas through a gas manifold 30 that is coupled to compression block 8 onthe cathode interconnect 22 side of ceramic cell 28 and/or on the anodeinterconnect 24 side of ceramic cell 28. Gas manifold 30 deliversoxidizing gas to metallic components and aluminum foil 4 inelectrochemical (e.g., SOFC) device 50 during heating of compressionassembly 100 in furnace 12 at conditions that form the protectivealumina coating layer on the surfaces of components in theelectrochemical device 50.

FIG. 3 is a scanning electron micrograph (SEM) showing an exemplaryalumina coating (protection) layer 14 formed on an exemplary alloy(AISI441) substrate 2 in accordance with the present invention. In thefigure, three spots (Spot #1-1) 60, (Spot #2-1) 62, (Spot #3-1) 64 ofsubstrate 2 were assayed and subjected to chemical analysis by EnergyDispersion Spectroscopy (EDS). TABLE 1 lists results of the chemicalanalyses.

TABLE 1 Chemical Analysis Results for Selected Spots of an aluminizedsurface analyzed by Energy Dispersion Spectroscopy (EDS) in accordancewith the present invention. Spot #1-1 Spot #2-1 Spot #3-1 Element (Atom%) (Atom %) (Atom %) O 59.73 62.03 57.72 Mg 0.73 — — Al 29.14 32.6437.46 Si 5.27 0.52 0.50 K 0.15 — — Cr 0.74 1.24 1.14 Fe 4.24 3.57 3.19

FIG. 4 is a scanning electron micrograph (SEM) showing an exemplaryalumina protection layer 12 formed on another (AISI441) alloy substrate2 in accordance with the present invention. Three spots (Spot #2-1) 66,(Spot #2-2) 68, (Spot #3-2) 70 of substrate 2 shown in the figure wereassayed and subjected to chemical analysis by Energy DispersionSpectroscopy (EDS). TABLE 2 lists results.

TABLE 2 Chemical Analysis Results for Selected Spots of an aluminizedsurface analyzed by Energy Dispersion Spectroscopy (EDS) in accordancewith the present invention. Spot #1-2 Spot #2-2 Spot #3-2 Element (Atom%) (Atom %) (Atom %) O 54.88 55.61 44.51 Al 37.55 34.58 32.97 Si 0.620.39 0.56 Nb — — 0.36 Cr 2.38 2.00 3.84 Fe 4.57 7.13 17.76

Spot analyses show that the protective alumina layer is formed on thesurface of the metal substrates. Other metal species (e.g., Cr) from themetal substrate are also present. In general, the alumina protectionlayers exhibit various morphologies and different penetration depths onthe surface of the metal (e.g., AISI441) substrates. The aluminaprotection layer adheres to the metal substrate and does not spallduring thermal cycling and operation.

The present invention can also be used to aluminize metal substrates,parts, and components in other than in-situ applications, e.g., wherecontinuous and protective alumina coatings are needed. Metallicsubstrates, parts, and components can be of any shape as long asaluminum foil can be applied to the surfaces of interest and acompression load can be delivered through an inert medium such as micapaper. Sources of aluminum can include foils as described herein, orlayers deposited by various processes such as, e.g., electroplating.

Applications

The present invention has numerous applications in the production ofhigh-temperature electrochemical devices, as well as applications inmanufacturing of coated items idea with uses in a wide range ofhigh-tech industrial manufacturing processes. The following Exampleprovides a further understanding of the invention.

EXAMPLE In-Situ Aluminization of Alloy Substrates

The compression assembly of FIG. 1 was used. Aluminum foil of athickness less than 25 um (microns) was sandwiched between a flat metalplate composed of, e.g., AISI441 steel (dimensions: 2″×2″×0.04″) and asheet of mica (e.g., 0.012″ thickness). The compression assembly wasfirst fired in air to 550° C. for 2 hours to burn off organic binders inthe mica sheet. Then, the compression assembly was heated to an elevatedtemperature of 900° C. for 2 hours to aluminize the metal substrate.

While exemplary embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its true scope and broader aspects. The appended claims aretherefore intended to cover all such changes and modifications as fallwithin the scope of the invention.

What is claimed is:
 1. A method for aluminizing a metal-containingsubstrate, the method comprising: compressing an aluminum foil of aselected thickness between one or more metal-containing substrates and arefractory material of a selected thickness with at least onecompression component under a selected compression load while heatingthe aluminum foil and the metal-containing substrate in an oxidizing gasat a selected temperature for a time sufficient to form an aluminumoxide coating layer on a surface of the one or more metal-containingsubstrates.
 2. The method of claim 1, wherein the process is performedon one or more metal-containing substrates of an electrochemical deviceselected from interconnects, cell frames, or combinations thereof priorto assembly in the electrochemical device.
 3. The method of claim 1,wherein the process is performed on one or more metal-containingsubstrates within an assembled device in-situ.
 4. The method of claim 1,wherein the process is performed on one or more metal-containingsubstrates within a stack assembly of an electrochemical device in-situ.5. The method of claim 1, wherein the compression load is greater thanor equal to about 6800 Newtons/m².
 6. The method of claim 1, wherein thecompression load is applied at the selected temperature for a timegreater than or equal to about 5 minutes.
 7. The method of claim 1,wherein the selected temperature is a temperature between about 660° C.and about 1200° C.
 8. The method of claim 1, wherein the heating isperformed in a furnace or a heater.
 9. The method of claim 1, whereinthe heating includes heating the metal-containing substrate to theselected temperature at a selected heating rate between about 1° C. perminute and about 10° C. per minute.
 10. The method of claim 1, whereinthe heating includes diffusing aluminum from the aluminum foil into thesurface of the metal-containing substrate.
 11. The method of claim 1,wherein the aluminum foil includes a thickness between about 0.001 mmand about 0.5 mm.
 12. The method of claim 1, wherein the process isperformed absent annealing at a selected temperature.
 13. The method ofclaim 1, wherein the aluminum oxide coating layer does not spall fromthe surface of the one or more metal-containing substrates in a deviceduring operation at an operating temperature.
 14. The method of claim 1,wherein the aluminum oxide layer reduces release of volatile metals fromthe one or more metal-containing substrates in a device during operationat an operation temperature compared to metal-containing substratesabsent the aluminum oxide layer.